1. Field of the Invention
The present invention relates to a speaker.
The present application claims priority from Japanese Patent Application No. 2005-50037, the disclosure of which is incorporated herein by reference,
2. Description of the Prior Art
A speaker provided with a dome-shaped diaphragm has merits that the size of speaker can be formed in a smaller diameter, and also that a good directivity is caused as known well. Accordingly, this type of speaker has been widely used as portable and personal sound sources.
It is well known that, in the speaker provided with the diaphragm having a dome shape at a part thereof, there occurs a disorder of frequency characteristics in which a sound pressure level of intermediate tone range becomes depressed by a reverse resonance between an edge portion and a central portion having a dome shape in the case where the edge portion is provided at an outer periphery of the diaphragm, and also an outermost fringe of the edge portion is attached on a frame in a structure of the diaphragm. In order to avoid such a disadvantage, there has been adopted a diaphragm called a “soft dome” which uses a material having a flexibility as one of characteristics of the material of the diaphragm. According to the speaker of such a “soft dome” type, the disorder of frequency characteristics in the intermediate tone range is restrained by designing not only to suppress a peak thereof as much as possible but also to keep a proper tone in an audible sound range through making use of vibrations at the edge portion.
The diaphragm of the “soft dome” type is usually formed by impregnating resin into cloth such as cotton, silk or synthetic fiber and then heating it, or is formed with foaming resin. Moreover, Japanese Unexamined Patent Application Publication No. Hei 08-242497 discloses a diaphragm formed by an olefinic elastomer denaturation nylon resin film.
FIG. 1 is an explanatory view showing a conventional speaker J1 provided with a diaphragm J2 having a dome-shaped portion J2a and an edge portion J2b formed on an outer periphery thereof, which is disclosed in Japanese Unexamined Patent Application Publication No. Hei 08-242497. An outermost fringe of the edge portion J2b is attached on a frame J3. The dome-shaped portion J2a is fixed on an upper portion of an outer periphery surface of a voice coil bobbin J4, and a voice coil J5 is wound up around a lower portion of an outer periphery surface of the voice coil bobbin J4. Further, a magnetic circuit is formed by an upper plate J6, a magnet J7 and a yoke J8. The voice coil J5 is located in a magnetic gap Jg formed between an inside surface of the upper plate J6 and an outside surface of a center pole portion J8a in the yoke J8.
A positioning accuracy of the voice coil J5 in the above-mentioned conventional speaker J1 is just dependent on support as caused by fixing the edge portion J2b on frame J3 and also a shape retaining function of the diaphragm J2 itself, so that it is very difficult to hold the voice coil J5 at an appropriate position in the magnetic gap Jg in an up-and-down direction in the case where the “soft dome” type of diaphragm having a low shape-retaining function is used, especially in which the edge portion J2b is widely formed, Moreover, the resonance frequency of the “soft dome” type of diaphragm J2 provided with the edge portion J2b is adjusted by the size of the edge portion J2b, and thus it is more difficult to hold the voice coil J5 at the appropriate position when the edge portion J2b is widely formed.
Especially, in the case where magnetic fluid is included in the magnetic gap Jg, there arises a problem that the voice coil J5 is not located at the appropriate position in the magnetic gap Jg due to a relation of a specific gravity of the magnetic fluid with those of both the voice coil bobbin J4 and the voice coil J5.
On the other hand, although it is possible to improve the shape retaining function of the diaphragm J2 itself by changing the materials thereof as described in the prior art mentioned above, a selection range of the materials is limited into a small range for doing so. In other words, the freedom degree of design in the frequency characteristic caused by any diaphragm materials is limited, Accordingly, extremely difficult is the design not only of desired frequency characteristics but also of sufficient shape retaining functions of the diaphragm J2 itself to be compatible with each other. In addition, a cost-up for manufacturing the diaphragm J2 may be unavoidable.
Followings are objects of the present invention for resolving the above-mentioned problems about the diaphragm of the soft dome type, particularly, provided with the edge portion:    {circle around (1)} Making it possible to appropriately position the voice coil;    {circle around (2)} Making it possible to adjust the frequency of the diaphragm by using a size of edge portion; and    {circle around (3)} Heightening the freedom degree of the characteristics design based on a material selection and making it possible to avoid a cost-up in manufacturing the diaphragm.